Straight from Microsoft Windows Phone developer platform team - Windows Phone Toolkit provides the developer community with new components, functionality, and an efficient way to help shape product development. WPToolkit releases include open source code,
samples & docs, plus design-time support for the Windows Phone platform.

FAQs -

Q - I have WP8 SDK and I installed the NuGet package, it is still installing the 7.1 version of the toolkit assembly, or I get an error – “You are trying to install this package into a project that targets 'WindowsPhone,Version=v8.0',
but the package does not contain any assembly references that are compatible with that framework.”

A – You have an older version of NuGet. Install the latest NuGet from
www.NuGet.org . Close and restart all the instances of Visual Studio. Now install the package. It will install correctly. WP8 projects are supported from Nuget 2.1 onwards.